BUG/MEDIUM: quic: fix sslconns on quic_conn alloc failure

BUG/MEDIUM: quic: fix sslconns on quic_conn alloc failure

QUIC connections are accounted inside global sslconns. As with QUIC
actconn, it suffered from a similar issue if an intermediary allocation
failed inside qc_new_conn().

Fix this similarly by moving increment operation inside qc_new_conn().
Increment and error path are now centralized and much easier to
validate.

The consequences are similar to the actconn fix : on memory allocation
global sslconns may wrap, this time blocking any future QUIC or SSL
connections on the process.

This must be backported up to 2.6.